1. Field of the Invention
The invention is generally directed to an adhesion promoting layer for bonding polymeric adhesives to metal and, more particularly, a heat sink assembly using the adhesion promoting layer as a thin adherent interfacial bonding layer between a multi-chip module cap and a polymeric adhesive used for attachment of a heat sink.
2. Description of the Related Art
Electronic devices such as computers rely on a large number of integrated circuits and other electronic components for their operation, most of which are mounted on printed circuit boards. Many of these components generate heat during normal operation. In order to accommodate increased demands for high operational speeds, a multi-chip module ("MCM") system has been developed and put into practical use. An MCM mounts a plurality of integrated circuit (IC) chips next to each other on a single substrate. This MCM arrangement can permit high density assembly and effectively reduces the lengths of connecting wires to endow the electronic circuit with increased operational speed.
However, as operational speed requirements increase, the amount of heat that the components must dissipate generally also will increase. Many components need the help of external heat sinks to dissipate the operational heat generated. The term "heat sink", as used herein, generally refers to a passive device, for example, an extruded aluminum plate with a plurality of fins or a water cooled cold plate, that is thermally coupled to an electronic component(s) to absorb heat from the component(s) and dissipate such absorbed heat into the air or water by convection. While the heat sink itself generally is passive in nature, it is to be understood that it is within the scope of this terminology to optionally direct a fan means at or attach a fan means to the heat sink fins to increase the rate of convective heat transfer, if desired, such as described in U.S. Pat. No. 5,396,403 (Patel).
As a matter of practical necessity, many components often will share a common heat sink where high packing density and/or small individual component sizes are involved. One approach to providing such a shared heat sink is described in U.S. Pat. No. 5,396,403 (Patel), mentioned above, which relates to a cooling structure for high power MCM systems using flip chip technology. In the Patel patent, the primary heat path is for heat to dissipate, in the following sequence, through the metallized back sides of high power chips having the heat generating components mounted on the opposite chip sides, through an interface of indium solder, through a thermally conductive plate of silicon carbide or a tungsten copper composite, through an interface of thermal paste, and lastly through a heat sink such as made of extruded aluminum. The heat sink described by the Patel patent is an integral structure which both constitutes a housing for the MCM and it is finned on its exterior to dissipate the heat into the surrounding air by convection. Yet, the need to form and align four separate and distinct thermal intermediary layers, mentioned above, in-between the chips and the heat sink without corrupting the device represents a relatively complicated and intensive manufacturing scheme for attaching the heat sink to the chips.
Another proposal, described in U.S. Pat. No. 5,387,815 (Nishiguchi), relates to a structure for cooling one or more high power flip chips in a semiconductor module. The stated objective of the Nishiguchi patent is to provide an excellent heat dissipation design with a small number of components. The Nishiguchi patent describes a semiconductor chip module having a substrate on which a wiring portion is formed, a semiconductor chip mounted so as to face a circuit side down to the wiring portion, a heat sink with an end in contact with a side opposite to the circuit side of the semiconductor chip, and a cap enclosing the semiconductor chip and having an opening exposing externally the other end of the heat sink. The Nishiguchi patent describes a metal film having a wetting property with respect to the solder to be used as being formed on the inner wall of the cap opening and on the tip and/or side surface of the heat sink which is inserted into the cap. An adhesive solder material is filled in the gap between the tip portion of the heat sink and the back side of the semiconductor chip, and adhesive solder material is also filled between the metal films on the inner wall of the cap opening and the side surface of the heat sink to hermetically seal the cap. However, the time and resources needed to be devoted to forming and precisely aligning cap openings and heat sinks inserted therethrough with chip surfaces, as well as concerns for device corruption due to solder overflow and the needed additional precautions taken in this respect, will restrict yield and reliability.
As another approach, a heat sink for thermal cooling has been attached with a filled silicone elastomer adhesive to the upper exterior side of a flat-topped MCM cap. The silicone elastomer adhesive used in this regard has been a silicone potting resin conventionally used in the electronic industry for encapsulating electronic assemblies and devices (e.g., the "Sylgard" trademarked series of silicone resins, made by Dow-Corning). The inner surface of this flat-topped MCM cap thermally communicates with chips housed in the cap via a thermal paste. Attachment of the heat sink to the outside of the MCM cap with the silicone resin has been accomplished directly to an anodized aluminum or a ceramic cap surface.
However, a problem arises when the attachment locus is a high performance MCM cap made of a highly thermal conductive material containing copper, such as copper tungsten (CuW). Usage of these copper-containing caps have become more relevant as thermal coefficient of expansion and thermal dissipation concerns change and evolve with ever increasing power needs. In order to attach a heat sink to the back surface of an MCM cap containing copper, such copper-containing caps have been metallized to provide and maintain a hermetically encapsulated package. A primary reason that such metallization is needed is attributable to the presence of an exposed continuous copper network in a composite such as copper tungsten, for example, which is susceptible to corrosion. The problem is heightened by the fact that contemporary MCM's often must maintain operational capability for ten years or more. One such metallization technique involves nickel plating the entire surface of the cap. The nickel-plated cap is selectively gold plated on the seal band area of the cap so that a hermetic solder seal (e.g., Pb/Sn) could be provided between the cap and a substrate supporting the chips.
However, the direct adhesive bond between the nickel plating on the cap and the silicone resin is inadequate as the nickel and the silicone have compatibility problems. Consequently, the nickel-silicone bond is prone to delamination and thus fails to meet current package performance and reliability requirements. However, the resort to bonding the highly conductive cap to a separate, discrete heat sink structure via the nickel to silicone resin bond is difficult to design around, especially where a combination of low thermal coefficient of expansion and high thermal conductivity are desired. For example, high cooling requirements tend to require a large (tall) heat sink. It would be very difficult and very costly, if not impossible, to fabricate an integral cap and heat sink with highly conductive cap materials such as either copper tungsten (CuW) or aluminum silicon carbide (AlSiC). In the case of CuW, the additional weight would also constitute a large liability (e.g., WCu is about six times as dense as aluminum).
From a more general perspective, the use of a thin layer containing chromium, zinc, or preferably a mixture of chromium and zinc, to enhance the adhesion between a lead frame and a polymeric molding resin, has been described in U.S. Pat. No. 5,343,073 (Parthasarathi). A mixture of chromium and zinc with a zinc-to-chromium ratio in excess of about 4:1 is described as most preferred. The lead frame is formed from an electrically conductive metal substrate.
It is also known to use a tantalum, titanium, or chromium film to promote adhesion between a metal substrate and a fluorocarbon film. Also, U.S. Pat. No. 4,582,564 (Shanefield) describes a method of forming adherent metal layers on certain epoxy substrates by providing a thin metal film on surfaces of the epoxy substrates after the substrates are pre-conditioned by sputter etching to remove weak boundary layers from the surface, and then depositing primary metal films over the thin adhesion-promoting base metal film. The thin adherent metal films are formed by vacuum depositing an adherent thin metal film of chromium, nickel, a nickel-vanadium alloy, platinum, palladium, or titanium in thicknesses from 50 to 10,000 .ANG.ngstroms, generally 1,000 .ANG.ngstroms, onto an epoxy surface. The Shanefield patent characterizes the use of the vacuum-deposited adhesion-promoting films as appearing to be unique for only rubber-modified epoxy and epoxies having a high degree of unsaturation in the polymer chain.
From the foregoing it will be apparent that there remains a need for a way to effectively bond a silicone elastomer adhesive to a metal having low adherability to that adhesive, such as nickel, and that there remains a particular need for a facile and less precision taxing approach to consolidate a heat sink assembly with an MCM in which a nickel-plated, highly thermal conductive MCM cap must be reliably bonded to a silicone elastomer, which, in turn, is used to join a heat sink to the cap.